1. Field of the Invention
This invention pertains generally to electrical distribution panels and, more particularly, to electrical distribution panels accommodating non-critical loads and critical loads being supplied with power from a second power source in response to power from a first power source becoming unacceptable.
2. Background Information
Electrical distribution panels, such as load centers, incorporate a plurality of circuit breakers and provide a safe and controllable distribution of electric power. Such load centers have become a common feature in both residential and commercial applications. Increasingly, such load centers are utilized in installations that incorporate, for example, an electric generator as a second power source in the event that a utility service serving as a first power source fails or becomes unacceptable.
With technological progress resulting in ever more uses for electricity, the amount of electrical power required for both residential and commercial applications has steadily increased, and this had lead to increased demand for relatively larger electric generators. Unfortunately, relatively larger electric generators present various disadvantages over relatively smaller electric generators. While smaller electric generators are typically air cooled, larger electric generators typically require a liquid cooling system with a circulation pump and radiator, thereby adding to both the costs and complexities of operating and maintaining a larger generator in comparison to a smaller generator. Larger generators also require relatively larger quantities of maintenance fluids, including lubrication oil and coolant liquid.
A known proposal for either new construction or the retrofit of existing installations involves the addition of a separate load center panel for critical circuits. This separate load center panel receives a backup power source and, also, manually switches between a utility power source and the backup power source. This can be utilized in installations that incorporate the backup power source, such as an electric generator, in the event that the utility power source becomes unacceptable (e.g., without limitation, fails; becomes unreliable; becomes unavailable). This can provide, for instance, reliable electric power for doctor's offices away from hospitals, home-based businesses and home-based chronic patient care support. For existing installations, this requires that the critical circuits be moved (e.g., rewired) from a first load center to the separate load center panel. However, it requires significant time and effort to rewire a load center in order to electrically connect a backup power source, such as an electric generator or other auxiliary power unit (APU) (e.g., a device whose purpose is to provide electrical energy), to critical circuits in, for example, residential and relatively smaller scale commercial structures.
Transfer switches are well known in the art. See, for example, U.S. Pat. Nos. 6,181,028; 5,397,868; 5,210,685; 4,894,796; and 4,747,061. Transfer switches operate, for example, to transfer a power consuming load from a circuit with a normal power supply to a circuit with an auxiliary power supply. Applications for transfer switches include stand-by applications, among others, in which the auxiliary power supply stands-by if the normal power supply should fail. Facilities having a critical requirement for continuous electric power, such as hospitals, certain plant processes, computer installations, and the like, have a standby power source, often a diesel generator. A transfer switch controls electrical connection of the utility lines and the generator to the facility load buses. In many installations, the transfer switch automatically starts the generator and electrically connects it to the load bus upon loss of utility power, and electrically reconnects the utility power source to the load bus if utility power is reestablished.
Another known proposal employs a single interlock between a main circuit breaker and a manual transfer switch. In response to loss of utility power, the user must first manually turn off any non-critical circuits, turn off the main circuit breaker, and then turn on the transfer switch. The manual sequence is reversed when utility power has returned.
Some known proposals provide mechanical interlocks between a main circuit breaker and a generator circuit breaker.
Other known proposals require that the entire load be switched from the utility power source to the generator power source. In other words, the loads are not separated into critical loads and non-critical loads.
U.S. patent application Ser. No. 12/043,514 discloses a first bus powered from a first circuit breaker and a first power input, a second bus, a transfer switch including a first input electrically connected to the first bus, a second input electrically connected to a second power input, and an output electrically connected to the second bus, and a number of pairs of circuit breakers. The number of pairs of circuit breakers include a second circuit breaker powered from the first bus, a third circuit breaker powered from the second bus, a power output powered from the second and third circuit breakers, and an interlock cooperating with the second and third circuit breakers and structured to prevent both of the second and third circuit breakers from being closed at the same time.
NEC 2008, Optional Standby Systems, provides in Section 702.5(2)(a)-(b) that where automatic transfer equipment is used, an optional standby system shall comply with either: (a) the standby source shall be capable of supplying the full load that is transferred by the automatic transfer equipment, or (b) where a system is employed that will automatically manage the electrically connected load, the standby source shall have a capacity sufficient to supply the maximum load that will be electrically connected by the load management system. Hence, for an automatic transfer switch, the standby system must be able to handle the entire load that is transferred.
There is room for improvement in electrical distribution panels.